The present invention relates to lead-acid storage batteries and, more particularly, to the treatment of pasted positive plates for such batteries to enhance the formation and initial performance efficiencies thereof.
In the preparation of the positive plates for a lead-acid battery, an active material paste is prepared from a mixture of lead oxide (PbO) containing a significant amount of metallic lead, sulfuric acid and water (along with various other well known additives). As a result of the chemical reaction during mixing, a portion of the lead and PbO is initially converted to lead sulfate (PbSO.sub.4) and the resultant positive active precursor paste comprises a heterogeneous mixture of lead, lead sulfate and basic lead sulfate (nPbO.PbSO.sub.4).
The precursor paste is applied to conductive lead grids and the freshly pasted plates are then typically cured to stabilize the precursor material and to enhance the strength and handleability of the plates for further processing. A cell element comprising a stack of alternately disposed positive plates and similarly prepared negative plates, between each of which is placed a separator, is then prepared. The cell elements are assembled in a battery container, interconnected in various manners well known in the art, and the assembled battery, filled with dilute sulfuric acid electrolyte, is electrochemically "formed" or initially charged by conversion of the positive and negative active precursor materials to lead dioxide (PbO.sub.2) and lead, respectively.
It is known that the formation efficiency of a lead-acid battery, that is, the extent of the conversion of the active precursor materials in relation to the applied current and duration of charge, is generally positive plate dependent. In particular, the conversion of PbSO.sub.4 to PbO.sub.2 in the positive active material is the most difficult to attain, due in part to the greater electrical potential required as compared to that needed for conversion of the other compounds in the positive paste materials. It is also known that in a typical pasted positive electrode, the formation of PbO.sub.2 from the heterogeneous paste mixture begins at the interface between the conductive lead grid and the paste and extends first into the cores of the paste pellets which fill the grid network. The conversion or formation eventually proceeds to the outer plate surfaces, that material being the last to be formed.
The inherent inefficiency of the formation process requires an actual formation charge greatly in excess of the theoretical, typically greater than 165% of the theoretical charge of 100 amp-hours/pound of positive paste material. The formation inefficiency is due primarily to the following factors: (i) the high resistivity of the positive active paste material; (ii) high current density at the conductor (grid)/paste interface which creates a potential sufficient to hydrolyze water and evolve hydrogen and oxygen; and (iii) temperature rises, especially during formation at higher current rates.
Notwithstanding the application of formation charges greatly in excess of the theoretical requirement, the typical formation of lead-acid batteries still often results in two serious deficiencies. First, the performance of lead-acid batteries immediately after formation (measured, for example, by the reserve discharge capacity) is often below the specified requirement. As a result, batteries must be "boosted" or cycled one or more times by discharge and recharge to bring them up to specified performance levels. Second, formed batteries often experience significant open circuit losses in capacity through self-discharge, even over relatively short periods. Both deficiencies are believed to be caused by the incomplete conversion of the PbO.PbSO.sub.4 paste material to PbO.sub.2 during formation.
The prior art discloses attempts to enhance the conductivity and hence the formation efficiency of positive active precursor pastes in lead-acid batteries. Thus, it is known to add conductive graphite fibers to the paste mix. However, graphite is unstable in the highly oxidizing environment of the positive electrode making the use of this material unsatisfactory. Another prior art approach is disclosed in U.S. Pat. No. 4,388,210 wherein ozone gas is used to generate a high surface area lead oxide and to oxidize a portion of the lead oxide in a hydrogen bonding solvent to form lead dioxide with increased surface area and electrical conductivity. The lead dioxide so produced is proposed for use as a component in preparing the active material paste, with the enhanced electrical conductivity promoting formation and the increased surface area improving active material utilization in the operation of the battery. However, in typical paste mixing procedure, the dilute sulfuric acid (H.sub.2 SO.sub.4) reacts with the lead dioxide particles to form surface layers of non-conductive lead sulfate thereon, thereby diminishing the electrical conductivity and the corresponding benefit in promoting formation.
U.S. Pat. No. 2,658,097 describes a method of treating pasted positive plates in an aqueous alkaline oxidizing solution to form on the plate surfaces a thin layer of lead oxide to promote accelerated formation and to allow the use of a higher specific gravity acid electrolyte for the formation process. However, the disclosed lower end-of-formation specific gravity of the forming electrolyte indicates that the positive plates are not completely formed, since it is well known in the art that a higher end-of-formation specific gravity is directly indicative of complete formation. In addition, the highly alkaline treatment solution has the potential to cause undesirable corrosion of the lead alloy plate grids.